MOLECULAR-DYNAMICS SIMULATIONS OF THE PHOTODISSOCIATION OF ICI ADSORBED ON A MGO(001) SURFACE

Molecular-dynamics (MD) simulations are used to study the vibrational properties of ICl adsorbed on an MgO(001) surface, and the photodissociation dynamics of the molecule after excitation to a 1Π electronic state. The electronic ground-state simulations show that ICl lies nearly parallel to the surface and occupies a single orientational site at surface temperatures below 150 K. Above 350 K the molecule hops between two orientational sites on the surface, and at 500 K full rotational diffusion of the adsorbate in the surface plane occurs. The multiplicity of sites and the onset of rotational diffusion at high T were found to greatly affect the dissociation dynamics and its temperature dependence. The photodissociation simulations show that only a fraction of the Cl atoms and some of the I atoms (which have a much higher binding energy) leave the surface following photolysis (at these energies). The fraction of Cl atoms leaving the surface subsequent to photodissociation at 50 K is ∼0.5, and it decreases as T is raised to 150 K. The trajectories show that Cl atoms leave the surface preferentially for initial ICl orientations in which the Cl end "points down." This orientation ensures that the escaping atom rapidly collides with the surface atoms. Momentum transfer due to surface local roughness is crucial for the Cl to acquire "escape velocity" normal to the surface. The angular intensity distribution of the Cl atoms is sensitive to surface corrugation, and the energy distribution of the photofragments strongly reflects the Cl/surface collision stage of the process. It is concluded that photodissociation experiments can provide information both on surface local structure and on photofragment/surface interaction and energy transfer. © 1990 American Institute of Physics.